1. Field of the Invention
This invention relates to an optical pickup apparatus for writing or reading information signals on or from a recording medium such as a magneto-optical disc. More particularly, it relates to an optical pickup apparatus in which a light beam radiated from a light source is radiated via an object lens to an information recording medium and in which the return light beam reflected from the information recording medium is detected by light detection means for reading out the information signals recorded on the information recording medium.
2. Description of the Prior art
An information recording medium for writing information signals by optical means has been proposed. A magneto-optical disc, typical of this type of the information recording medium, is made up of a disc substrate, and a signal recording layer. The signal recording layer is formed of a material having its direction of magnetization inverted by heating it to a temperature higher than the Curie temperature and applying a corresponding magnetic field. An optical pickup apparatus is employed for writing and reading information signals on or from the information recording medium.
Referring to FIG. 1, a conventional optical pickup apparatus is made up of a laser diode device 1, as a light source, and an object lens 9, adapted for converging the light flux radiated from the laser diode element 1 on signal recording layer 21 of disc 23. The optical pickup apparatus is also provided with a photodetector 12 for detecting the light flux converged on and reflected from the signal recording layer.
The laser diode device 1 is provided with a can-shaped package and radiates light upon being supplied with current from plural terminals. The laser device 1 includes a semiconductor chip arranged in the package and radiates light from a terminal end of an active layer of the semiconductor chip. The laser diode device is a point source of light. The light flux radiated from laser diode device 1 is collimated by a collimator lens 7 before being incident on a polarized beam splitter 5. The object lens 9 converges the light flux on a signal recording surface which is a boundary surface between a signal recording layer 21 of a magneto-optical disc 23 and a disc substrate 20.
On the signal recording surface, the information signals are written on spirally extending substantially concentric recording tracks.
The light flux converged on the signal recording surface is reflected by the signal recording surface so as to be re-incident as a reflected light flux on the object lens 9. The reflected light flux is collimated by lens 9 before being re-incident on the polarized beam splitter 5. The reflected light flux re-incident on the polarized beam splitter 5 is reflected for the most part by a semi-transmitting film 6 within polarized beam splitter 5 so as to be incident on a three-beam Wollaston prism 11. The semi-transmitting film 6 of the polarized beam splitter 5 is so designed that, of the reflected light flux, about 95% of the S-polarized light is reflected by the semi-transmitting film 6 and about 80% of the P-polarized light component is transmitted through the semi-transmitting film 6.
The reflected light flux incident on three-beam Wollaston prism 11 is transmitted through prism 11 so as to be converged by a condenser lens 22 on a light receiving surface of the photodetector 12.
The photodetector 12 has plural light receiving surfaces and is provided within a package. The photodetector 12 is designed to output the intensity of the fluxes of the light received by the light receiving surfaces as electrical signals. The electrical signals outputted from the photodetector 12 are signals which are indicative of detected changes in the intensity of the reflected light flux, changes in the polarization of said light, or changes in the extent of its astigmatism.
For writing information signals on magneto-optical disc 23, the optical pickup device converges the flux of light radiated from the laser diode device 1 on the signal recording layer 21 for locally heating the signal recording layer 21. At this time, an external magnetic field is applied to the signal recording layer 21 by a magnetic device 17, as shown in FIG. 1. The state of light flux convergence on the signal recording layer 21 and adjustment of the position of radiation of the light flux on the signal recording layer 21 may also be adjusted at this time based on the various information produced on the basis of electrical signals outputted from photodetector 12.
Information signals may be read by the optical pickup device from magneto-optical disc 23 on the basis of the information derived from photodetector device 12 while the light flux radiated by the laser diode element is converged and radiated on the signal recording layer 21.
In the above-described optical pickup device, the polarized beam splitter 5 is arranged at a point on a light path between the laser diode device 1 and the object lens 9 where the light flux is a collimated light beam. The three-beam Wollaston prism 11 is also so arranged that collimated light flux reflected by reflective film 6 of polarized beam splitter 5 is incident on prism 11.
This is because if polarized beam splitter 5 were arranged in a converging light flux or in a diverging diffused light flux, there would be a tendency to produce double refraction at reflective film 6 of polarized beam splitter 5. If double refraction should be produced at reflective film 6, the so-called CN ratio is lowered to render it impossible to read out information signals from the magneto-optical disc 23 satisfactorily. The double refraction can be produced because the semi-transmitting film 6 of the polarization beam splitter 5 has incident angle dependent incident light flux transmittance so that phase differences are produced at the semi-transmitting film 6 between the P-polarization component and the S-Polarization component of the converging or diverging light fluxes.
In a read-only optical pickup apparatus, to reduce the size and the structure of the apparatus, the beam splitter can be placed at a position on a light path of the reflected light flux from the optical disc which the reflected light flux is a converging light beam. If the beam splitter is arranged in a converging light flux, the void space between laser diode device 1 and collimator lens 7 may be utilized efficiently and converging lens 22 may be eliminated to render it possible to reduce the size and the structure of the optical pickup apparatus. In other words, the fact that the polarized beam splitter 5 has to be provided in a collimated light flux hinders attempts to achieve a simpler structure and a smaller size of the optical pickup apparatus.
There is disclosed in U.S. Pat. No. 4,771,414 an optical pickup apparatus in which a three-beam Wollaston prism is used for introducing a light beam reflected from a signal recording surface of a magneto-optical disc into a photodetector.